1. Field of the Invention
The present invention relates to an apparatus for expanding and controlling sound fields designed to correct the asymmetry of sound fields that will occur as in an automotive vehicle compartment when stereophonic signals are reproduced by loudspeakers disposed laterally asymmetrically relative to a listening position, and to expand the expanse of the sound fields for stereo-sound reproduction with presence.
2. Description of the Prior Art
FIG. 1 (1) is a plan view explanatory of asymmetric sound fields formed within a vehicle compartment 51. In automotive stereo-sound reproducing apparatuses, as FIG. 1 (1) shows, in a vehicle compartment 51, a right-channel loudspeaker sr is disposed at a front right position of a driver's seat 52, while a left-channel loudspeaker sl is disposed at a front left position of a side seat 53. These loudspeakers sl, sr are built, for example, in an instrument panel 54.
In a typical prior-art arrangement, the loudspeakers sl and sr are supplied with acoustic signals from a sound signal source as adjusted in only right- and left-side balance, that is level.
Therefore, when sounds of equal energy level are released from the loudspeakers sl and sr, at the position of driver 55 as shown, the acoustic energy distribution on the hearing sense of the driver 55, as FIG. 1 (2) shows, is not uniform between the left and right loudspeakers sl and sr and tends to become biased toward the loudspeaker sr which is nearer to the driver 55.
Accordingly, the localization position of virtual sound source that should primarily be localized in the frontward direction of the driver 55 indicated by reference character l51 becomes biased toward the loudspeaker sr indicated by reference character 57. Even when the adjustment of the above mentioned balance is made, the acoustic energy distribution cannot be balanced between the right side and the left side, and the angle of lateral divergence or bias of the sound fields cannot be corrected.
With prior art automotive stereo-sound reproducing apparatuses, therefore, the problem is that the direction of localization of an sound image is deviated from the forward direction to form an asymmetrical sound image and this prevents sound reproduction full of presence.
An approach toward solving the foregoing problem is described in U.S. Pat. No. 4,866,776. According to this prior art disclosure, a center loudspeaker sc is disposed between loudspeakers sl and sr of left- and right-side channels on an instrument panel 54. At the center loudspeaker sc, added signals comprising acoustic signals of left- and right-side channels are converted into acoustic vibrations.
Through this arrangement, at a right side seat 51 as viewed in the forward direction of vehicle compartment 51, sound fields are formed by the right-channel loudspeaker sr and center loudspeaker sc, while at a left-side seat 53, sound fields are formed by the left-channel loudspeaker sl and center loudspeaker sc. Thus, sound fields that are comparatively well balanced between the right- and left-channels are formed at both the right-side and left-side seats.
In this prior art arrangement, however, the right-channel loudspeaker sr is disposed at angle .theta.51 relative to the frontward direction shown by reference character l51, whereas the center loudspeaker sc is disposed angle .theta.52 which angle is wider than the angle .theta.51. Therefore, the sound which the driver 55 hears involves some deviation in phase as pointed out above according to the difference in distance between the listening position of the driver and the respective loudspeakers sr, sc.
Another problem is that in the limited space of the compartment 51, because of the limitations as to the mounting positions of the loudspeakers sl, sr, the angle of divergence shown by reference character .theta.51 is smaller than 30 degrees, an angle which can form an ideal sound field. At the position of the driver 55, this is such that the direction of the source of the right-channel sound cannot be localized outwardly of the loudspeaker sr which is disposed at a comparatively narrow angle of divergence. Therefore, the sound field is very narrow and provides no satisfactory presence.
Such a problem occurs likewise with a television receiver in which the right and left loudspeakers are narrowly spaced. When the viewer moves away from the screen to a location suitable for viewing the screen, the angle of divergence becomes very narrow because of the narrow distance between the two loudspeakers and the viewer cannot enjoy good presence.
Another prior art arrangement which is intended to overcome this deficiency is disclosed in U.S. Pat. No. 4,953,219. In this prior art disclosure, a delay period for formation of reverberation sounds is selected on the basis of reverberation time within the vehicle compartment 51 that has been previously measured, whereby reverberation sounds of a generally acceptable level may be produced to compensate for a lack of presence.
However, with reverberation sounds only, no wide distribution of fundamental sounds such as vocal sounds can be obtained, it being thus difficult to improve the sense of presence to any satisfactory extent.
Another prior art arrangement intended to solve the above problem is disclosed in Japanese Patent Publication JP 1-40560. According to this prior art arrangement, reverberation sounds are added and, in addition, it is arranged that at the seat position of the driver 55, for example, the acoustic signal of the right channel for the loudspeaker sr as adjusted in phase and level are output from the loudspeaker sl, whereby some good result can be obtained which is equivalent to that obtainable in the case where the right channel loudspeaker sr is disposed at a position indicated by reference character sra. In this way, an improved sense of presence has been achieved through the expanding of the sound fields and the addition of reverberation sounds.
In this prior art arrangement, however, respective sound fields of acoustic signals of fundamental sounds from acoustic signal sources, such as a magnetic tape reproducing device and a radio receiver, and of acoustic signals of added reverberation sounds are collectively expanded and, therefore, sound images of vocal sounds and the like cannot be localized in the frontward direction.